The science of containerless processing of materials under the microgravity conditions of orbital space flight has raised the need for specialized instrumentation. This includes noncontact positioning and furnace systems, remote sample handling techniques, and noninvasive diagnostic instrumentation. Of the latter, the remote measurement of specimen temperatures is recognized as being particularly important.
In a typical containerless processing application, a specimen is positioned inside a hypothetical furnace. Acoustic or electromagnetic forces are used to maintain this position. The specimen itself is invariably spherical, possibly molten, and may be a metal or glass, but in any case, it generally has a highly specular surface. The furnace may have walls which are heated so as to radiantly heat the specimen. Alternatively, the specimen may be heated by some external source such as a laser and, at high temperatures, its own incandescence will illuminate the inside of the furnace. In either case the amount of background radiance in the furnace will be significant.
Thermocouples are clearly unsuitable for measuring the temperature of the specimen in such cases, and radiometric pyrometry is a logical choice. By measuring the radiance, R, of the target, and if its emissivity, .epsilon., is known, then the temperature may be determined from the Planck radiation law, ##EQU1## where C.sub.1 and C.sub.2 are the first and second radiation constants and .lambda. is the wavelength of the measurement. This is a well-documented and useful technique and the exponential relationship enables good resolution to be achieved.
However, there are some specialized considerations for containerless processing that need to be addressed in order to obtain true temperatures from radiance measurements. Not the least of these is the problem of the background radiance which can contribute to the measured radiance levels resulting in an error in the inferred temperature. Indeed, if a metal is to be melted in a heated-wall furnace, the effective emissivity of the closed furnace cavity may be so large that the walls contribute more radiant energy than the specimen itself. For example, if the furnace acts like a blackbody but the specimen emissivity is only 0.3, for example, then the furnace will generate about three times the radiant energy intensity of the target. Since the target is specular, this energy may be reflected into the pyrometer. From Equation (1), it is easy to show that at 1000.degree. C., for example, the error in the inferred temperature will be about 200.degree. C. for measurements at 650 nm, a wavelength typical of many pyrometers. It will be difficult to implement corrections of this magnitude that preserve the accuracy and confidence needed in the measurement.